There is worldwide, as well as national interest, in the development of a hydrogen economy. Potentially the most efficient, cost effective, and large scale means of obtaining hydrogen is from thermochemical methods.
In a thermochemical process, heat plus water yields hydrogen and oxygen. All other chemicals within the process are fully recycled. While more than 100 such thermochemical cycles have been identified, only a few are considered potentially viable.
To be practical, a thermochemical cycle must be efficient, be non-polluting, involve relatively few chemical reactions, and have acceptable capital costs. The leading candidates are the sulfur cycles (sulfur iodine (SI) and hybrid sulfur (HS)). While these processes have desirable characteristics, they have three highly undesirable characteristics: (1) the operating temperature is ˜850° C. with pressure of ˜10 atm, (2) because of the presence of pressurized, corrosive iodine and concentrated sulfuric acid, the materials of construction will be very expensive, and (3) the processes have significant inventories of pressurized, hot, hazardous volatile chemical reagents.
The SI operating of ˜850° C. is at the very limits of practical engineering materials. Lowering the peak temperatures by 100 to 200 degrees would significantly improve process viability. In addition, the chemical reagents used in these processes are also a concern because they are highly toxic volatile dense gases that, in case of an accident, could travel off-site at ground level.
The current two candidate thermochemical processes are the sulfur-iodine (SI) process and the hybrid sulfur (HS) cycles. These processes require heat input at ˜850° C. if the process operates at ˜10 atm. The highly endothermic (heat-absorbing) gas-phase reactions in each of these processes are2H2SO4⇄2H2O+2SO3⇄2SO2+2H2O+O2 (850° C.)  (Eq. 1)
These two thermochemical processes have other lower-temperature chemical reactions. The SI process has two other lower temperature chemical reactions (equations 2 and 3), which, when combined with the reaction in equation 1, (1) yield H2 and O2 from water and heat and (2) recycle all other chemical reagents.I2+SO2+2H2O ⇄2HI+H2SO4 (120° C.)  (Eq. 2)HI⇄I2+H2 (450° C.)  (Eq. 3)
The HS process (also known as Westinghouse, GA-22, and Ispra Mark 11) has a single low-voltage electrochemical step (Eq. 4) that is needed to complete the cycle.SO2(aq)+2H2O(1)⇄H2SO4(aq)+H2(g) (Electrolysis: 80° C.)  (Eq. 4)
The greatest practical challenge of the sulfur process is the high temperatures required for each of the sulfur iodine and hybrid sulfur cycles. In each of these cycles, the high-temperature sulfur trioxide (SO3) dissociation reaction (Eq. 1) is an equilibrium chemical reaction that requires a catalyst. High temperatures and high pressures are required to drive the reaction towards completion and acceptable yields. Detailed studies have concluded that the required minimum temperatures need to be very high (825 to 850° C.) to drive the SO3 decomposition to near completion.
After the high-temperature dissociation reaction, all the chemicals must be cooled to near room temperature, the oxygen separated out and released to the atmosphere, the SO2 sent to the next chemical reaction, and the unreacted sulfuric acid (formed by recombination of SO3 and H2O at lower temperatures) reheated to high temperatures. Unless the chemical reactions go almost to completion, the energy losses in separations and the heat exchangers to heat and cool all the unreacted reagents result in a very inefficient and uneconomical process. An analysis of the SI flowsheet showed that process efficiencies decreased very rapidly with decreasing temperature resulting in incomplete dissociation of SO3, such that the process could not produce H2 below 700° C.
The efficiency of a thermochemical process depends on the reactions' yields and the energy losses associated with the processing equipment. Higher temperatures result in higher heat losses from the plant and higher pressures require more energy for the compression and pumping. Therefore, lowering the temperatures and pressures of thermochemical cycle generally improves the overall thermal efficiency of the process. Also, the release of dense toxic gases that can spread offsite is a threat to public safety. The SI and SH processes both have large inventories of hot, pressurized hazardous gases that are heavier than air and capable of spreading along the ground and incurring off-site injuries and deaths. Also, these methods for generating hydrogen from water use many steps. Fewer steps lower the capital and operating costs and result in higher efficiencies.
Therefore, there is a need for a new method for generating hydrogen that can be preformed at a lower temperature and pressure, using nonvolatile and/or less toxic reactants that require a only a minimum number of process steps and readily available equipment.